The present invention relates to a hydraulic system for a motor vehicle.
Various concepts are known for the drive of a hydraulic fan motor and a hydraulic steering motor. One concept is based on providing two completely separate hydraulic fluid circuits for the drive of a fan and a hydraulic steering device and therefore also two pumps. The two pumps can be substantially integrated in one another, and are then referred to together as a tandem pump. Despite the extensive integration such a tandem pump is more expensive than a single hydraulic pump, such as used in motor vehicles for instance for the supplying of hydraulic fluid to only a power-assisted steering device.
In another design, known from DE-OS 28 50 481, for the driving of a hydraulic steering motor and a hydraulic fan motor only a single hydraulic pump is employed. Behind the hydraulic pump there is a priority valve which provides assurance that, up to a limit value, the entire amount of hydraulic fluid delivered by the hydraulic pump flows to the steering device. Only the excess can be fed to the fan motor. With such a design of a hydraulic system, a pump having a large piston displacement is necessary since, in addition to the quantity pumped which flows continuously to the steering device, an additional amount must be produced for the fan motor. Furthermore a priority valve is required so that a hydraulic system in accordance with DE-OS 28 50 481 is elaborate and costly.
Finally, from U.S. Pat. No. 3,664,129, a hydraulic system is known in which the fan motor is arranged in series with the steering device between the latter and a hydraulic pump. As in the case of the other embodiments sketched, the embodiment in accordance with the U.S. patent also has a bypass valve by which the division of the fluid flow through the fan motor and the bypass line circumventing the fan motor is variable as a function of the cooling power required. If the cooling power required increases, the bypass valve is adjusted in such a manner that the flow of hydraulic fluid through the bypass line is reduced. Conversely, the flow of hydraulic fluid through the bypass line increases when the cooling power required decreases. In principle, a two-position control is also possible in this connection such that the bypass line is entirely open or entirely closed. In the hydraulic system of U.S. Pat. No. 3,664,129, the hydraulic pump is to be able to cover the added pressure requirement of the hydraulic steering device and the fan motor regardless of the amount thereof, and is to be developed accordingly. Only for the limiting of parasitic power losses of the internal combustion engine which drives the hydraulic pump is it provided that, as from a given pump pressure, the pressure drop over the fan motor is reduced so that the pump pressure would only rise again when there is no longer a pressure drop over the fan motor and the fan motor is at a standstill. For the reduction of the pressure drop over the fan motor as from a given pump pressure, a pressure-limiting valve which is set to said pump pressure has its inlet connected to the outlet of the hydraulic pump, which at the same time is the inlet of the fan motor and has its outlet connected to the outlet of the fan motor.